In the normal combustion of a fuel-air mixture in an internal combustion engine, the fuel-air mixture is ignited by an ignition spark and then burns in a controlled and progressive manner as the flame front advances through the compressed fuel-air mixture in the cylinder chamber. However, an undesired auto-ignition and uncontrolled explosion of the as-yet un-burned fuel-air mixture, commonly known as "knocking", "pinging", or "detonation", can also occur. Such knocking is generally to be avoided because it causes intense pressure waves oscillating in the cylinder, which cause a vibration of the engine components and a resultant audible knocking noise. Ultimately, the intense knocking forces can damage or destroy the engine. Nonetheless, under at least some operating conditions, an engine can achieve its maximum power output and efficiency by operating directly at the limit or boundary of knocking conditions. Thus, an engine controller, such as the conventionally known electronic control unit (ECU), aims to operate the engine as close as possible to the knocking limit without actually causing knocking. If knocking does occur, then corrective measures are taken, for example the ignition timing is slowly retarded, i.e. adjusted in a direction toward the top dead-center position of the piston.
Knocking combustion is generally characterized by pressure oscillations having a frequency in the range from 5 to 20 kHz, taking place in a time interval following the maximum cylinder pressure, i.e. maximum compression. The knocking combustion can be detected by measuring and evaluating changes in the ionic current that flows within the cylinder combustion chamber. This ionic current can be sensed by a suitable sensor arranged in the cylinder, for example a spark plug may be used as an ionic current sensor. However, the ionic current signal already exhibits a first maximum signal level as well as oscillations of the ionic current at the time when the flame front advances and spreads through the fuel-air mixture in the cylinder. This maximum signal level and oscillations in the ionic current can be mis-evaluated to result in an erroneous detection of knocking when knocking has not actually occurred, because these signal oscillations are caused by turbulence in the cylinder and not by knocking.
Methods are known in the art for trying to detect the occurrence of knocking combustion by analyzing the ionic current signal. However, such known methods have been found to be especially disadvantageous and ineffective, particularly when atypical operating conditions of the engine arise, or whenever such methods are to be used in specialized engines, for example engines using a multiple spark ignition during a single combustion cycle. In these situations, the known knock detection methods often give erroneous results. Erroneous results, and especially false detections of knocking, can also arise when the ignition timing is adjusted, for example in order to avoid knocking.
More specifically, the prior art methods in this context involve comparing the signal level of the ionic current signal to a single minimum knocking threshold. Namely, if the signal level of the ionic current signal exceeds the minimum knocking threshold, then the prior art methods indicate that knocking combustion is taking place. However, such methods fail to take into account certain characteristics of the knocking phenomenon. First, the time portion of the ionic current signal that is significant for recognition of the knocking combustion is relatively brief, and therefore a failure to properly limit the sampling or evaluation of the signal to a pertinent time window leads to erroneous knock recognition. Secondly, and simultaneously, very strong oscillations of the ionic current signal can be caused, for example especially by the flame front expanding through the fuel-air mixture during the normal or proper combustion process. Since these strong oscillations are not caused by knocking but rather by normal combustion influences, the prior art methods that fail to distinguish between these oscillations and the signal variations stemming from knocking are prone to erroneous knock recognition. Namely, this characteristic oscillation of the ionic current signal will often result in a signal level that exceeds the minimum knocking threshold, and is therefore falsely recognized as a knocking phenomenon.
For the above reasons, in all of the above mentioned cases, the known methods can result in the detection of at least a portion of the expanding flame front in the measuring time window during which the ionic current signal is measured or evaluated. Since the flame front causes a strong variation in the ionic current signal, the result is an erroneous detection of knocking combustion. Accordingly, any combustion process in which the expanding flame front influences the ionic current sensor during the sampling and evaluation time window will be recognized as a knocking combustion, and an anti-knock control will be activated as is known from German Patent 4,239,592, for example. The anti-knock control will responsively shift the ignition timing back toward top dead-center, and as a result, during subsequent combustion cycles the flame front will expand even more strongly into the measuring time window during which the ionic current is sampled and evaluated. This will result in the detection of an even more strongly varied ionic current signal, which will accordingly be recognized as an even worse knocking condition, whereupon the anti-knock control will be actuated to retard the ignition timing point even further. This chain of events leads to a complete breakdown or failure reaction of the ignition timing control due to the initial erroneous recognition of knocking combustion when knocking was not actually occurring.